Investigation has been conducted to remove particulate matter discharged from diesel engines, by using particulate-matter-capturing ceramic honeycomb filters comprising ceramic honeycomb structures having porous cell walls, through which an exhaust gas containing particulate matter passes, namely diesel particulate filters (DPFs). As shown in FIG. 1, the ceramic honeycomb filter 11 comprises pluralities of flow paths 15a, 15b partitioned by porous cell walls 14, a peripheral wall 11a formed around the flow paths 15a, 15b, and plugs 13a, 13b alternately formed in end portions of the flow paths 15a, 15b. As shown in FIG. 1(b), an exhaust gas containing particulate matter flows into the flow paths 15b open on an exhaust gas inlet-side end surface 12a, passes through cell walls 14, and flows out of the flow paths 15a open on an exhaust gas outlet-side end surface 12b, during which particulate matter in the exhaust gas is captured by fine pores (not shown) in the cell walls 14.
When the captured particulate matter is excessively accumulated in a ceramic honeycomb filter, the pressure loss of the filter increases, likely resulting in power decrease of the engine. Accordingly, the captured particulate matter is periodically burned by an external ignition means such as an electric heater, a burner, etc., to regenerate the ceramic honeycomb filter. A set of two ceramic honeycomb filters are usually mounted for an alternate regeneration system, in which one is used while the other one is regenerated.
With respect to the characteristics of a honeycomb filter having the above structure, it is important to keep the pressure loss of a filter low to avoid decrease in engine performance. It is also required that the honeycomb filter has enough resistance to withstand heat shock due to rapid temperature change during regeneration, engine stop, etc. Thus, technologies for improving plugs of ceramic honeycomb filters have been disclosed so far, as described below.
As a technology of plugging end surfaces of a ceramic honeycomb structure at predetermined positions, JP 63-28875 B discloses a method for plugging open end surfaces of a ceramic honeycomb structure, comprising plugging ends of flow paths in a sintered honeycomb structure with a cordierite-forming material paste, and then sintering the cordierite-forming material to cordierite at a temperature of 1300° C. or higher. This method achieves complete plugging of flow paths of a ceramic honeycomb structure at open end surfaces, providing a highly reliable cordierite honeycomb filter having excellent heat shock resistance.
JP 2002-136817 A discloses a ceramic honeycomb filter obtained by plugging flow path ends of a sintered or unsintered ceramic honeycomb structure with a pulverized sintered or unsintered plugging material, which has the same composition as that of the ceramic honeycomb structure, and heating the plugging material at a high temperature of 1400° C. to form plugs at flow path ends of the ceramic honeycomb structure. It further describes that because plugs at flow path ends of this ceramic honeycomb filter are made of the same material as that of the ceramic honeycomb structure, the ceramic honeycomb structure and the plugs do not suffer cracking due to their thermal expansion difference and are free from troubles such as the peeling of plugs, when used under high-temperature conditions.
However, the ceramic honeycomb structure likely has a small thermal expansion coefficient because a cordierite-forming material is oriented by extrusion, while a cordierite-forming material is not substantially oriented in plugs. Accordingly, in the technologies described in JP 63-28875 B and JP 2002-136817 A, it is difficult to provide the ceramic honeycomb structure and the plugs with completely the same thermal expansion coefficient. Further, because the plugs are fused to the sintered ceramic honeycomb structure at high temperatures of 1300° C. or higher, large residual stress is generated after fusion. Thus, heat shock by an exhaust gas, and mechanical shock by engine vibration and road vibration, cracking occurs in plugs, their boundaries with the honeycomb structure, etc., likely resulting in the peeling of plugs while using the filter.
To solve such problems, JP 2005-125318 A discloses a ceramic honeycomb filter comprising porous cell walls defining flow paths for removing particulate matter from an exhaust gas, which is obtained by forming plugs in predetermined flow paths of a ceramic honeycomb structure made of a material comprising cordierite as a main crystal; at least part of the plugs being formed by at least ceramic particles and an amorphous oxide matrix of colloidal oxide. JP 2005-125318 A describes that this honeycomb filter is obtained by bonding the plugs to the ceramic honeycomb structure at 1000° C. or lower. According to this invention, there is little difference in a thermal expansion coefficient between the plugs comprising at least ceramic particles and the ceramic honeycomb structure, and a bonding temperature lowered by using an amorphous oxide matrix of colloidal oxide leaves less residual stress in the ceramic honeycomb structure, resulting in a ceramic honeycomb filter having excellent heat shock resistance. In addition, the low bonding temperature is effective to drastically reduce a production cost.
Though the ceramic honeycomb filter of JP 2005-125318 A used as a particulate-matter-capturing filter has excellent heat shock resistance, it has been found when it is used as a ceramic honeycomb filter carrying a catalyst such as an oxidation catalyst for accelerating the oxidation (combustion) of captured particulate matter, which may be called “catalyst-carrying filter” below), the temperature of a filter substrate is elevated by combustion, reducing the bonding strength of the plugs to cell walls, so that the plugs may be detached, resulting in low particulate-matter-capturing performance.
JP 2015-505748 A discloses a method for forming plugs hardenable at low temperatures without sintering by charging a aqueous composition comprising a refractory filler comprising coarse cordierite particles having a narrow particle size distribution with d50 of 10-40 μm, an inorganic binder, and a binder, into a ceramic honeycomb body. It describes that the plugs are provided with less recesses when the aqueous composition charged into the ceramic honeycomb body is dried. JP 2015-505748 A describes that larger particle sizes in the filler reduce shrinkage and the overall movement of a composition in flow paths or fine pores, thereby reducing the number of recesses.
However, it has been found that the method described in JP 2015-505748 A does not provide sufficient bonding strength, when the aqueous composition charged into the ceramic honeycomb body is dried, for example, in a hot-air furnace.